Fix Notify widget crash on finalise

If `dbus-next` is not installed the `Notify` widget will crash on
`finalize` as it tries to call `notifier.unregister` but `notifier` is
`None`. This has a knock-on effect of `widgets_map` not being cleared
and additional widgets not being finalised.

libqtile/widget/notify.py

@@ -219,5 +219,6 @@ def invoke(self):
             self._invoke()
 
     def finalize(self):
-        notifier.unregister(self.update, on_close=self.on_close)
+        if notifier is not None:
+            notifier.unregister(self.update, on_close=self.on_close)
         base._TextBox.finalize(self)